Functional Electrical Stimulation Combined With Exercise for Rotator Cuff-Related Shoulder Pain in Older Adults

Subacromial shoulder pain is common in older adults and is associated with rotator cuff disease. The rotator cuff muscles stabilize the humeral head against the upward pull of the deltoid during arm elevation. Previous work from our laboratory showed that intermuscular EMG-EMG coherence between rotator cuff muscles increases during fatigue in healthy individuals but not in individuals with subacromial pain, suggesting impaired common neural drive to these muscles. Functional electrical stimulation (FES) has been shown to increase corticospinal excitability and muscle activation, but its long-term effects on neuromuscular coordination of the shoulder have not been studied.

This randomized controlled trial evaluates whether a 4-week resistance training program combined with FES improves neuromuscular control of the supraspinatus, infraspinatus, and middle deltoid compared with the same training without FES. Neuromuscular control is assessed using surface and intramuscular (fine-wire) EMG during isometric shoulder abduction at 25%, 50%, and 75% of maximal voluntary contraction, during ramp contractions, and during a sustained fatiguing contraction. Acromiohumeral distance and supraspinatus tendon occupation ratio are measured with ultrasound at 0°, 45°, and 60° of shoulder abduction.

• Functional electrical stimulation (Device), also known as NEUBIE, FES, Pulsed Direct Current Electrical Stimulation: Pulsed direct current electrical stimulation delivered by the NEUBIE stimulator through surface electrodes placed over the supraspinatus and infraspinatus muscles during resistance exercises (40 Hz, 0.46 ms pulse width, maximum comfortable intensity).
• Sham Functional Electrical Stimulation (Device), also known as Sham FES: Identical electrode placement over the supraspinatus and infraspinatus and identical NEUBIE stimulator setup as the active FES intervention, applied during the same resistance exercises, with stimulator intensity set to 0 so that no electrical current is delivered.
• Progressive Resistance Training (Behavioral): A standardized 3-phase progressive resistance training program (Michener et al., 2024) using elastic resistance bands, performed 3 times per week for 4 weeks under supervision by research staff. Participants progress through phases based on performance and symptom tolerance.

Primary outcomes

• Intermuscular EMG Coherence During Submaximal Isometric Contractions [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: EMG-EMG coherence between muscle pairs (supraspinatus-infraspinatus, supraspinatus-middle deltoid, infraspinatus-middle deltoid) recorded with surface and fine-wire electromyography during 30-second sustained isometric shoulder scaption at 25%, 50%, and 75% of maximal voluntary contraction. Coherence is quantified in the delta (2-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) frequency bands. Coherence values range from 0 (no shared neural drive) to 1 (complete shared neural drive).
• Intermuscular EMG Coherence During a Fatiguing Contraction [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: EMG-EMG coherence between muscle pairs (supraspinatus-infraspinatus, supraspinatus-middle deltoid, infraspinatus-middle deltoid) in the delta (2-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) frequency bands during a sustained isometric contraction at 25% of maximal voluntary contraction held until the endurance limit (force drop of 5% MVC). Values range from 0 (no shared neural drive) to 1 (complete shared neural drive).

Secondary outcomes

• Acromiohumeral Distance [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Shortest linear distance between the inferior edge of the acromion and the humeral head, measured with ultrasound at rest (0° abduction) and during active shoulder abduction at 50% MVC at 45° and 60°. Each value is the average of 3 measurements. Measured in centimeters.
• Supraspinatus Tendon Occupation Ratio [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Ratio of supraspinatus tendon thickness to acromiohumeral distance, calculated as (tendon thickness / AHD) × 100, measured with ultrasound at 0°, 45°, and 60° of shoulder abduction. Each value is the average of 3 measurements. Expressed as a percentage.
• Shoulder Pain and Disability Index (SPADI) [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Self-administered questionnaire assessing shoulder pain and difficulty with upper-extremity activities of daily living. Total score ranges from 0 to 100, with higher scores indicating greater pain and disability.
• Muscle Fatigue Rate [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Slope of the EMG median frequency over time for the supraspinatus, infraspinatus, and middle deltoid during the sustained fatiguing contraction, calculated from surface and fine-wire EMG. A steeper negative slope indicates faster fatigue.
• Muscle Onset Time During Ramp Contractions [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Onset time of the supraspinatus, infraspinatus, and middle deltoid relative to force onset during isometric shoulder abduction ramp contractions (0 to 100% MVC at 7.5% MVC/s). Onset is defined as EMG amplitude exceeding 3 standard deviations of resting baseline for at least 25 ms. Measured in milliseconds.
• Motor Unit Recruitment Threshold [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Force level at which individual motor units in the supraspinatus and middle deltoid are recruited during ramp contractions, identified from fine-wire EMG as the first of four consecutive spikes. Expressed as percentage of maximal voluntary contraction (% MVC).
• Motor Unit Initial Firing Rate [Baseline, 2 weeks (mid-intervention), 4 weeks (end of intervention), and 8 weeks (4 weeks after the end of intervention)]: Firing rate of individual supraspinatus and middle deltoid motor units at recruitment during ramp contractions, calculated as the average of the first three inter-spike intervals. Measured in hertz (Hz).

Sex: All, ages: 60 Years to any

Inclusion Criteria:

* Having pain more than 2 weeks in the anterolateral shoulder region
* A painful arc of movement (60°-120°)
* Having shown positive impingement signs, including positive Neer, Hawkins, and Empty Can tests

Exclusion Criteria:

* Acute diagnosed rotator cuff tears
* Osteoarthritis-induced shoulder pain
* Currently receiving treatment or physical therapy for shoulder pain
* Previous shoulder surgery
* Taking medications that affect the central nervous system
* Metabolic or cardiovascular disease
* Metal Implants or pacemakers
* Skin lesions on the shoulder
* Aversion to needles